Our objective is to evaluate the developmental capability and the progressive restriction of potential of the neural crest-derived cells that migrate to an colonize the avian enteric mesenchyme. Segments of developing quail gut, containing primordia of enteric ganglia, will be grafted into appropriate (vagal) and inappropriate (truncal) crest migration pathways (back-transplantation) of younger chick embryonic hosts. Under these conditions certain neural crest-derived precursor cells recover the ability of their ancestors to migrate along crest migratory pathways and colonize distant tissues and organs. The phenotypes expressed by the back-transplanted cells, made recognizable in the experimental animals by the quail nuclear marker, will be identified. Specifically, we will ascertain whether back-transplanted precursors, derived from the bowel, will form enteric-type neurons and glia after they migrate to ectopic loci or, if instead, they will give rise to end stage cells that are the targets of the migratory pathways into which the donor cells are placed. Controls, done to determine whether cells emigrating from back-transplanted bowel, are of crest or mesodermal origin will include grafting hindgut, which has not yet been colonized by cells from the neural crest, and chimeric foregut, prepared so that only crest cells carry the quail marker. In addition, doubly labeled chimeric bowel, containing quail crest cells and fluorescent chick mesodermal cells will also be back-transplanted. This will permit the migration of crest- and mesoderm-derived components of the enteric mesenchyme to be traced simultaneously. Monoclonal antibody markers will also be utilized. Results will be compared with those of experiments in which ciliary ganglia are grafted into the vagal pathway leading to colonization of the gut. Finally, experiments will be done to determine when the neuroblasts of the developing quail and chick bowel cease dividing. The possibility that this schedule can be changed by back-transplanting the developing ENS into neural crest migration pathways will then be evaluated. These experiments will determine if there are still precursor cells in the ENS with neurogenic, gliogenic, or mesectodermal potential after cell division has finished in situ, and if that potential can be revealed by back-transplantation to a crest migratory pathway. The ENS is a unique part of the PNS with many features that resemble the brain. Principles derived from studies of its development may therefore be more relevant to the CNS than studies of other peripheral ganglia.